The present invention relates to model locomotives or railway engines and, more particularly, to scale model steam locomotives.
Some steam locomotives which are reproduced as models had two independent sets of steam cylinders and driving wheels. In some cases, such locomotives were articulated so that they could more easily negotiate curves, whereas others were provided with two sets of cylinders and driving wheels mounted in a rigid frame. An example of the articulated construction was the Union Pacific 4-8-8-4 heavy freight locomotive, known as the "Big Boy". An example of the rigid construction was the Pennsylvania Railroad 4-4-4-4 heavy passenger locomotive known as a "duplex" type.
In both constructions, the real locomotives had no connection between the two sets of cylinders and driving wheels, save that in some cases the locomotives were arranged so that steam was compounded, being used first in one set of cylinders and then in the other. However, in neither construction were the two sets of driving wheels mechanically coupled together so that the wheels of one set were permanently linked in a predetermined angular configuration to the wheels of the other set. This meant that the wheels of one set would, in fact, alter their angular position with respect to the wheels of the other set owing to track irregularities, the propensity of one or other set of wheels to slip, etc.
Models of the above types of locomotives have often been produced with a single electric motor driving both sets of driving wheels through appropriate shafts and gearing. With this arrangement, there is no capability for the wheels of one set of assume a different angular position with respect to the wheels of the other set, other than through back-lash and play in the gears and couplings, which is negligible. In another arrangement, it has been proposed to use two electric motors, one for each individual set of driving wheels, so that the wheels of each set are able to assume different angular positions from time to time.